A differential nutrient enrichment bioassay with Skeletonema costatum showed that phytoplankton in the continental Dutch coastal waters was potentially phosphorus and silicon limited in 1988. In the central North Sea, potential N limitation predominated. The bioassay results depended on the ratio of the dissolved nutrients in the test water and on the optimum atomic ratio for the test diatom species. The nutrient concentrations were often so low that actual limitations of nutrient uptake rates were possible. In the stratified region of the North Sea dinoflagellates and µ- flagellates were dominant, while diatoms were absent in June and July in that area. In the mixed parts of the study area both diatoms and flagellates were present. The absence of diatoms during the summer stratification in the central North Sea cannot be explained by Si limitation. It seems more probable that it is caused by a combination of N depletion and losses from sedimentation.